Systems that convert between kinetic energy and electrical energy sometimes exhibit large output voltage fluctuations, especially if no mechanical energy storage device is used. In a conventional piston-cylinder device, for example, a flywheel can maintain a nearly constant crankshaft speed, even though the extraction of energy is discontinuous by nature. For example, in the context of a conventional piston engine, energy is transferred to the crankshaft during an expansion stroke, while it is removed from the crankshaft during a compression stroke. Conventional engines can therefore be connected to a constant power load, such as an electric motor, without any additional device to manage the energy transfer. Systems that do not include a flywheel or equivalent device, such as a free-piston machine, for example, operate with discontinuous energy transfers.
Systems that are connected to an electrical grid must provide power that meets the requirements of the grid. Large fluctuations, on a DC bus coupled to a system, for example, place operational demands on a corresponding grid tie inverter (GTI). Additionally, operation of the system is constrained to limit fluctuations of voltage of the DC bus.